Re: [PATCH] mm,oom_reaper: don't call mmput_async() without atomic_inc_not_zero()

[Michal Hocko <mhocko@suse.cz>]

On Thu 02-06-16 21:20:03, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Wed 01-06-16 15:53:13, Andrew Morton wrote:
[...]
> > > Is it even possible to hit that race? 
> > 
> > It is, we can have a concurrent mmput followed by mmdrop.
> > 
> > > find_lock_task_mm() takes some
> > > care to prevent a NULL ->mm.  But I guess a concurrent mmput() doesn't
> > > require task_lock().  Kinda makes me wonder what's the point in even
> > > having find_lock_task_mm() if its guarantee on ->mm is useless...
> > 
> > find_lock_task_mm makes sure that the mm stays non-NULL while we hold
> > the lock. We have to do all the necessary pinning while holding it.
> > atomic_inc_not_zero will guarantee we are not racing with the finall
> > mmput.
> > 
> > Does that make more sense now?
> 
> what Andrew wanted to confirm is "how can it be possible that
> mm->mm_users < 1 when there is a tsk with tsk->mm != NULL", isn't it?
> 
> Indeed, find_lock_task_mm() returns a tsk where tsk->mm != NULL with
> tsk->alloc_lock held. Therefore, tsk->mm != NULL implies mm->mm_users > 0
> until we release tsk->alloc_lock , and we can do
> 
>  	p = find_lock_task_mm(tsk);
>  	if (!p)
>  		goto unlock_oom;
>  
>  	mm = p->mm;
> -	if (!atomic_inc_not_zero(&mm->mm_users)) {
> -		task_unlock(p);
> -		goto unlock_oom;
> -	}
> +	atomic_inc(&mm->mm_users);
>  
>  	task_unlock(p);
> 
> in __oom_reap_task() (unless I'm missing something).

OK, I guess you are right. Care to send a patch? That also means that
your patch to set mm = NULL in the atomic_inc_not_zero path is not
really needed and in fact e2fe14564d3316d1625ed20bf1083995f4960893 which
is sitting in the Linus tree is OK.

I will comment on the rest in a separate reply to not mix the two
things.
-- 
Michal Hocko
SUSE Labs

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